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Broadly speaking, when it comes to war, there are three moral positions available to us.

We can adopt the pacifist position of rejecting the morality of war at all; the realist position that sees war not as a moral enterprise, but as a function of amoral national interest; or the just war position that, under certain conditions, war may be morally permissible, or even necessary.

Most people, even if unaware of it, adopt a form of Just War Theory (JWT), which has informed international law and is frequently invoked by political leaders in discussions of war.

JWT has a long history. In the West it can be traced back at least as far as St Augustine of Hippo, who lived in the 4th century, though there are traces of these ideas in Aristotle’s writing as well. Traditionally, JWT comprises of three separate areas: jus ad bellum, jus in bello, and jus post bellum, each of which corresponds to a different aspect of the ethics of war: the pre-war considerations, the conduct of war, and the resolution of conflict.

Pre-war considerations

Jus ad bellum is concerned with those matters that are necessary in order for the decision to go to war to be a morally good or permissible one. For this reason, it has typically been considered the domain of political leaders. Jus ad bellum comprises of several conditions, each of which have to be satisfied for it to be morally justifiable to engage in war.

The conditions include: just cause, right intention, legitimate authority, and proportionality and probability of success.

Just cause is usually considered the most important criterion of jus ad bellum. It requires, simply, that political communities may wage war only in response to particular serious wrongdoing, and only in response to serious wrongdoing.

The most common justification for war is the crime of aggression—the violation of the political sovereignty or territorial integrity of another political community. (Indeed, recent movements have argued that states have a responsibility to protect the victims of aggression).

Right intention requires that the reasons that justify the war also be the reasons why the political leaders commit to military action. This matters mainly because to accept the huge moral harms of war for some lesser goal is disproportionate and vicious, but also because wrongly-intended wars are more likely to justify other moral violations.

Legitimate authority is a procedural requirement that aims to minimise the frequency with which war occurs, and regulate its declaration by requiring that only the authorised leaders of political communities can authorise the resort to war.

Proportionality and probability of success recognise the harm that even just wars can inflict on the global community and on the innocent. It therefore requires that political leaders can reasonably anticipate that the general, impartial state of affairs will be better off if war is engaged in rather than if it is not.

Disproportionate wars cause more harm on the general community than they prevent, and this is no more true than in cases where war is fought against impossible odds. Thus, war can only be just if we can reasonably anticipate achieving success (although success may mean different things in different contexts).

Last resort is also based in the recognition that war should not be engaged in unless all other reasonable diplomatic options, sanctions, and other less severe measures have been exhausted. This is again predicated on the belief that war punishes not only the guilty but the innocent as well.

Conduct of war

Jus in bello regards the morally correct ways in which war can be waged. It is therefore seen as more obviously relevant to warfighters and their officers than to politicians, although the political leadership should never encourage or authorise actions (such as torture) that violate in bello norms.

The first of the principles that JWT insists be upheld in the conflict of war is that of discrimination. Crucially, warfighters may never intentionally attack a person who is innocent. Conventionally, this has been understood to mean that only enemy combatants may be targeted, and that civilians must never be intentionally harmed.

But there has been some recent debate to suggest that only unjust combatants (that is, combatants fighting for an unjust cause) are legitimate targets of attack. The argument is that those who are fighting for a just cause have done nothing wrong; that is, nothing to forfeit their right against being killed. In the relevant sense, so the argument goes, they are innocent.

Regardless of one’s position in this debate, it is universally agreed that civilians ought not to be intentionally targeted.

But JWT holds that an attack that aims to kill legitimate targets but also kills civilians as an unintended side-effect may be morally acceptable. The idea here is that collateral damage may at times be inescapable in war, and that when collateral damage is reasonably guarded against but still cannot be avoided, the attackers are not morally responsible for those deaths because they were unintended. The deaths are, morally speaking, tragic, but not ones that a person ought to be punished for.

And finally, there’s the question of proportionality. As with the ad bellum version, proportionality requires that the force used by warfighters not be excessive to the (morally acceptable) goals of their operation. It prevents the use of unnecessarily harmful force.

A military response to the deaths of three Israeli youths last year might have been justified, but the overwhelming missile strikes authorised by Israeli Prime Minister Binyamin Netanyahu were demonstrably disproportionate.

Resolution of conflict

Jus post bellum is a new and growing area of scholarship (although some thinkers, such as Immanuel Kant, considered the subject hundreds of years ago), is a response to recent post-war experiences.

This approach aims to provide a moral framework for the transition from the state of war to a continuing state of peace; in particular, following an intervention by a third party into another conflict.

New as it is, the precise conditions (and the specifics of those conditions) are not universally accepted, but a primary area of debate concerns the question of whether post-war resolution should be “backward-looking”—focused on restoring the status quo after war, punishing wrongdoers, compensation of victims, and redistributing rights appropriately, or “forward-looking,” and concerned with how to create a just and lasting peace.

It is likely that a robust jus post bellum will address some elements of both these concerns, but difficulties arise quickly. Without appropriate international legal mechanisms and financial support for the victims of injustice, the enforcement of strict responsibilities on interveners is likely to discourage states from intervening when otherwise they might.

Note that I’ve said nothing about some areas of real controversy: torture, armed drones, preventive war, humanitarian intervention, child soldiers or a range of other issues.

But the claim of JWT is that the framework it provides will assist us in identifying some of the possible moral issues with any of these.